House roofs should never exceed three layers of shingles. Building and city codes require roofers to limit tile layers to two. At most, you can safely lay up to two layers of organic asphalt or fiberglass roofing shingles with an inclination of up to 18.5 degrees. If there are more than two layers, too much weight is added to your roof.
Some local building codes don't allow overlaps; in those municipalities, you can only have one layer of shingles. It's always a good idea to review the applicable codes before starting a repair or renovation project. New layers of shingles can be added to a roof, but there are more factors that need to be considered. Overlapping can have its benefits, and it can also inadvertently cause damage to a new layer of shingles if the older layer was not properly inspected.
The roof must be able to support the weight of two layers of shingles, as well as be able to support the additional weight in winter due to a layer of snow, without the frame being stressed to the point of breakage. Your contractor's labor warranty will remain the same, but you will only have the standard material warranty for your architectural shingles. To make matters worse, a roof with two layers of shingles retains more heat, which accelerates deterioration. In many cases, a repair may be needed, and if the roof was replaced within a period of 10 to 15 years, you may not need a complete replacement of the roof tiles.
But if your current asphalt roof is a good candidate for a new spiked roof, there are a few things you should keep in mind before jumping straight into your project. To make a nailing, an asphalt roof has to be near the end of its useful life and the shingles must lie fairly flat (no raised or cracked shingles, blisters, or bumps). A new roof is placed on a flat surface and should not show drops and elevations as dramatic as a layer would on top of them, especially if high-strength shingles are not used for the second layer. Once the roof has been stocked, workers lift several layers of shingles at once and throw them into plastic garbage cans (see Figure).
Two layers of shingles trap more heat, which in turn damages the shingles and shortens the life of the roof. This can give you time to organize your budget for removing and reattaching the roof of your home in about five to ten years if you have roof damage and can't afford a full roof tile replacement at that time. If someone decides to replace or remove the old layer under the new one, then you will have to tear down both layers, which can cost an exorbitant amount of money. Some property inspectors also point out that a double-layer roof is a negative and some potential buyers may be discouraged if the roof looks messy even with the new layer of shingles.
If your asphalt tile roof is near the end of its useful life and the shingles are flat (no shingles, blisters, or raised or cracked bumps), your roof could be a good candidate for nailing it. However, a reputable roofing contractor will recommend a full replacement if you already have two or more layers.